1. FIELD OF THE INVENTION
The present invention relates to a method of recovering exhaust gas energy of an internal combustion engine such as a gasoline engine, and, more particularly, to a method of and an apparatus for recovering exhaust gas energy of an internal combustion engine which are suitable for use in an automobile.
2. DESCRIPTION OF THE PRIOR ART
Heat engines are in general not efficient. Even in a relatively efficient internal combustion engine, more than half of consumed energy is discarded as waste.
Accordingly, method of recovering exhaust gas energy so as to enable efficient use of the discarded energy was proposed in, for example, the Japanese Patent Application Laid-Open No. 57-59010.
In this known method, a turbine generator is driven by exhaust gas energy, and a motor directly connected to an internal combustion engine is in turn driven by the power of this generator so as to achieve energy recovery. However, the laid-open publication does not disclose a concrete mean of controlling the motor by the power of the generator: it is not taken into consideration in this method.
In another type of conventional technique such as those disclosed in Japanese Patent Publication No. 53-46242, control system for diving an electric motor in constant or reduced torque characteristics was also known. The control of the electric motor directly connected to the internal combustion engine, however, was not disclosed concretely, either. It is not taken into consideration either.
In an engine used in an automobile, the engine speed (rotation speed) N thereof and the torque T.sub.E of the output shaft thereof in general have a relationship such as that shown by a curve A in FIG. 4. As can be seen from the curve A, the engine has a small torque T.sub.E at a low speed area Al, and the torque T.sub.E thereof slightly reduces at an area A2 at which the engine speed N is extremely high.
Further, as shown by curves B1 and B2 in FIG. 5 whose abscissas and ordinates respectively represent the engine speed N and the rate D of engine operating time at each engine speed N, the engine is used most of the time around an engine speed nl (for example 2,000 to 3000 r.p.m) which is substantially one-half of the maximum engine speed Nmax (about 6000 to 7000 r.p.m in the case of a gasoline engine) or less. A hatched area B3 defined by the curves B1 and B2 denotes variations in the rate D of engine operating time at each engine speed N.
More specifically, the curve B1 corresponds to the rate D in a state where the throttle valve of the engine is nearly closed, while the curve B2 corresponds to the rate D in a state where the throttle valve of the engine is largely opened. As the amount of energy recovered can be defined by a product of the engine-operating time rate D of engine-operating time at a rotation speed N and the output or power of the motor at the speed N, the efficient of energy recovery can be made higher, when the power of the motor is made higher at an area where the rate D is higher.
In the above-described simple aggregation of the known methods of recovering exhaust gas energy, however, the area which exhibits high engine-operating time rate D and the area at which energy can be recovered highly efficiently do not coincide.
If the electric motor is operated or driven at a constant torque characteristics or mode which is most generally employed in the operation thereof, an effective power recovery is not effected, and these known methods suffer from the following disadvantages:
A very high torque cannot be obtained at a low speed area at which the largest torque is required, precluding a sufficient degree of power recovery.
Despite the fact that the required torque becomes relatively small as the maximum engine speed Nmax approaches, the motor is driven at a high power, increasing its loss (caused by non-load loss) and reducing its efficiency remarkably. At the same time, a raised temperature of the motor reduces a life thereof and hence a reliability.
At the area which exhibits a relatively high engine operating time rate D, e.g., at the area of an engine speed which is one-half to one-thirds of the maximum engine speed Nmax or less, sufficient power recovery is not effected. At an area of a high engine speed N, less torque is generally required. These operating conditions result in utilization of only half of the capacity or power of the electric motor.
In this specification, the term "efficiency" may be referred to as a ratio or proportion of energy transmitted to the output shaft of the engine with respect to the energy supplied by the combustion of the fuel in the engine averaged over time during operation thereof, taking the ratio D into consideration. In addition, the term "efficiency" may also be referred to as a proportion of the energy utilized to drive the output shaft of the engine or power by the motor with respect to the energy generated by the generator in considering or evaluating the energy recovery at the high speed area.